1. Field of the Invention
The present invention relates to a common-mode feedback circuit.
2. Description of the Prior Art
The purpose of a common-mode feedback circuit is to stabilize an associated fully differential amplifier. A stable fully differential amplifier can only be achieved when the bandwidth of the common-mode feedback circuit is greater than the bandwidth of the fully differential amplifier. Also, the common-mode feedback circuit needs to be stable as well. Instability within a common-mode feedback circuit is caused by, for example, high impedance nodes. Traditionally, capacitors are used to compensate for high impedance nodes, but the addition of capacitors decreases the bandwidth of the common-mode feedback circuit, and, thus, places a restriction on the bandwidth of the fully differential amplifier.
A common-mode feedback circuit according to the present invention includes a converting circuit converting the output voltages of a fully differential amplifier into currents, and a summation circuit summing the currents to produce a summation current. The summation current is then compared by a comparison circuit to reference current. A feedback circuit generates a feedback voltage for controlling the fully differential amplifier based on the results of the comparison. Advantageously, the common mode feedback circuit according to the present invention does not include any high impedance nodes or suffer from the problems and disadvantages associated therewith.